1. Field of the Invention
The invention relates to synthetic oligonucleotides that are useful for studies of gene expression and in the antisense oligonucleotide therapeutic approach. More particularly, the invention relates to synthetic oligonucleotides that have improved qualities for such applications resulting from modifications in the sugar phosphate backbone of the oligonucleotides.
2. Summary of the Related Art
The potential for the development of an antisense oligonucleotide therapeutic approach was first suggested in three articles published in 1977 and 1978. Paterson et al., Proc. Natl. Acad. Sci. USA 74: 4370–4374 (1977) discloses that cell-free translation of mRNA can be inhibited by the binding of an oligonucleotide complementary to the mRNA. Zamecnik and Stephenson, Proc. Natl. Acad. Sci. USA 75: 280–284 and 285–288 (1978) discloses that a 13-mer synthetic oligonucleotide that is complementary to a part of the Rous sarcoma virus (RSV) genome inhibits RSV replication in infected chicken fibroblasts and inhibits RSV-mediated transformation of primary chick fibroblasts into malignant sarcoma cells.
These early indications that synthetic oligonucleotides can be used to inhibit virus propagation and neoplasia have been followed by the use of synthetic oligonucleotides to inhibit a wide variety of viruses. Goodchild et al., U.S. Pat. No. 4,806,463 (the teachings of which are hereby incorporated by reference) discloses inhibition of human immunodeficiency virus (HIV) by synthetic oligodeoxynucleotides complementary to various regions of the HIV genome. Leiter et al., Proc. Natl. Acad. Sci. USA 87: 3430–3434 (1990) discloses inhibition of influenza virus by synthetic oligonucleotides. Agris et al. Biochemistry 25: 6268–6275 (1986) discloses the use of synthetic oligonucleotides to inhibit vesicular stomatitis virus (VSV). Gao et al., Antimicrob. Agents Chem. 34 808–812 (1990) discloses inhibition of herpes simplex virus by synthetic oligonucleotides. Birg et al., Nucleic Acids Res. 18: 2901–2908 (1990) discloses inhibition of simian virus (SV40) by synthetic oligonucleotides. Storey et al., Nucleic Acids Res. 19: 4109–4114 (1991) discloses inhibition of human papilloma virus (HPV) by synthetic oligonucleotides. The use of synthetic oligonucleotides and their analogs as antiviral agents has recently been extensively reviewed by Agrawal, Trends in Biotech 10: 152–158 (1992).
In addition, synthetic oligonucleotides have been used to inhibit a variety of non-viral pathogens, as well as to selectively inhibit the expression of certain cellular genes. Thus, the utility of synthetic oligonucleotides as agents to inhibit virus propagation, propagation of non-viral pathogens and selective expression of cellular genes has been well established. However, there is a need for improved oligonucicotides that have greater efficacy in inhibiting such viruses, pathogens and selective gene expression. Various investigators have attempted to meet this need by preparing and testing oligonucleotides having modifications in their internucleoside linkages. Several investigations have shown that such modified oligonucleotides are more effective than their unmodified counterparts. Sarin et al., Proc. Natl. Acad. Sci. USA 85: 7448–7451 (1988) teaches that oligodeoxynucleoside methylphosphonates are more active as inhibitors of HIV-1 than conventional oligodeoxynucieotides. Agrawal et al., Proc. Natl. Acad. Sci. USA 85: 7079–7083 (1988) teaches that oligonucleotide phosphorothioates and certain oligonucleotide phosphoramidates are more effective at inhibiting HIV-1 than conventional oligodeoxynucleotides. Agrawal et al., Proc. Natl. Acad. Sci. USA 86: 7790–7794 (1989) discloses the advantage of oligonucleotide phosphorothioates in inhibiting HIV-1 in early and chronically infected cells.
In addition, chimeric oligonucleotides having more than one type of internucleoside linkage within the oligonucleotide have been developed. Chimeric oligonucleotides contain deoxyribonucleosides only, but have regions containing different internucleoside linkages Pederson et al., U.S. Pat. No. 5,149,797, the teachings of which are hereby incorporated by reference, discloses chimeric oligonucleotides having an oligonucleotide phosphodiester or oligonucleotide phosphorothioate core sequence flanked by oligonucleotide, methylphosphonates or phosphoramidates. Furdon et al., Nucleic Acids Res. 17: 9193–9204 (1989) discloses chimeric oligonucleotides having regions of oligonucleotide phosphodiesters in addition to either oligonucleotide phosphorothioate or methylphosphonate regions. Quartin et al., Nucleic Acids Res. 17-7523–7562 (1989) discloses chimeric oligonucleotides having regions of oligonucleotide phosphodiesters and oligonucleotide methylphosphonates. Each of the above compounds contains deoxyribonucleotide phosphorothioates, which have reduced duplex stability. Atabekov et al., FEBS Letters 232: 96–98 (1988) discloses chimeric oligonucleotides in which all internucleoside linkages are phosphodiester linkages, but in which regions of ribonucleotides and deoxyribonucleotides are mixed. Inoue et al., FEBS Letters, 215: 237–250 (1987) discloses chimeric oligonucleotides having only phosphodiester linkages, and regions of deoxyribonucleotides and 2′-OMe-ribonucleotides. None of these compounds having solely phosphodiester linkages exhibit either endonuclease or exonuclease resistance.
Many of these modified oligonucleotides have contributed to improving the potential efficacy of the antisense oligonucleotide therapeutic approach. However, certain deficiencies remain in the known oligonucleotides, and these deficiencies can limit the effectiveness of such oligonucleotides as therapeutic agents. Wickstrom, J. Biochem. Biophys. Methods 13: 97–102 (1986) teaches that oligonucleotide phosphodiesters are susceptible to nuclease-mediated degradation. Such nuclease susceptibility can limit the bioavailability of oligonucleotides in vivo. Agrawal et al., Proc. Natl. Acad. Sci. USA 87: 1401–1405 (1990) teaches that oligonucleotide phosphoramidates or methylphosphonates when hybridized to RNA do not activate RNase H, the activation of which can be important to the function of antisense oligonucleotides. Agrawal et al., Nucleosides & Nucleotides 8: 5–6 (1989) teaches that oligodeoxyribonucleotide phosphorothioates have reduced duplex stability when hybridized to RNA.
There is, therefore, a need for improved oligonucleotides that overcome the deficiencies of oligonucleotides that are known in the art. Ideally, such oligonucleotides should be resistant to nucleolytic degradation, should form stable duplexes with RNA, and should activate RNase H when hybridized with RNA.